Keep it. I suppose it could be referring to the natural oil of the hair/scalp, or it could be referring to some kind of hair treatment oil which you've bought. Perhaps the expression is suggesting that you not do anything which might remove that oil.
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Anonymouswhat is retain the oil on the scalp?This is not an intelligible sentence. You need to rewrite it because you have two verbs, "is" and "retain", which do you want to use?